docs: def assets_by_site(self):
          """
          :returns: numpy array of lists with the assets by each site
          """
          assets_by_site = [[] for sid in range(self.tot_sites)]
          for i, ass in enumerate(self.array):
              assets_by_site[ass['site_id']].append(self[i])
          return numpy.array(assets_by_site)